1) Field of the Invention
The field of this invention relates to flowmeters and more particularly to a linear true mass flowmeter to determine directly the mass flow rate of a fluid within a conduit.
2) Description of the Prior Art
A mass flow rate meter is an instrument that measures the mass of a fluid flowing through a conduit, which includes pipes and ducts, per unit time. There have been many attempts to develop practical industrial mass flowmeters. A common type of flowmeter is the orifice sensor type. A plurality of sensors, such as four in number, are located within the path of the flowing fluid. By measuring the flow through each of the orifices, the mass flow rate can be calculated utilizing a hydraulic Wheatstone bridge.
In order to obtain the linear true mass flow rate of the fluid, a differential flow across the sensors must be obtained. In order to achieve this, a recirculating pump is utilized that will extract fluid from one of the sensors and supply that same volume of fluid to another of the sensors. This is what occurs when four in number of sensors are utilized, that fluid will be extracted from two of the sensors and supplied to the remaining two sensors. In order to achieve a true mass flow measurement, an arrangement of these sensors is required that adds and subtracts a recirculating flow to the main flow so that the difference between the signals from each sensor will result in a linear true mass flow measurement.
Although the use of orifice plates in mass flowmeters has long been known, other types of differential pressure flow sensors could be utilized such as a venturi, flow nozzle, elbow meter, pitot tube, target sensor and laminar flow element. The present invention is discussed in particular in relation to a target sensor. However, it is considered to be within the scope of this invention that any other type of differential pressure sensor could be utilized.
As mentioned previously, the mass flowmeter of the prior art utilizes four sensors in conjunction with a hydraulic Wheatstone bridge. From a manufacturing standpoint, the hydraulic Wheatstone bridge (HWB) flowmeter is difficult to build and therefore expensive. To achieve high accuracy, the four orifices of this type of flowmeter must be matched to extremely high accuracy. Typically HWB meters are accurate to one-fourth to one-half of a percent with a rangeability of 100:1 to 200:1. This high accuracy requires, among other things, that the flow through each of the orifice plates be extremely closely matched over a wide flow range. This matching is primarily determined by the discharge coefficient and the area of each orifice. Typically each orifice must be precision ground and then individually tested so that a matched set of orifices can be obtained.
Another limitation of the HWB meter is that it is only suitable for clean liquids without gas bubbles in the flow stream. This is because debris and bubbles in the liquid will build up in front of the orifice plates and change their discharge coefficients. Also, because the recirculating pumps used have close tolerances, these pumps cannot pump debris without causing damage to the pump which affects accuracy or in the case of a severe damage, renders the pump (and therefore the meter) inoperable.
Another limitation of the HWB meter for large flow applications is that all the main flow must pass through the meter. This requires that the recirculating pump be sized to pump more than the highest main flow rate. In other words, it is not possible to make an insertion probe type HWB meter that measures the mass flow in a portion of the main flow stream with the majority of the main flow going around the sensor. Insertion probe flowmeters are useful for large flow measuring applications such as large diameter pipes.
Another limitation of the HWB meter is that it is not suitable for low-cost applications. The differential pressure produced across a HWB meter is typically low. High accuracy transducers for low differential pressures are expensive.